Thread: How well does your Isuzu tow??

Originally Posted by Ancient Mariner

Any thing you can do to reduce heat in a transmission has to be a plus and torque being transferred thru a straight shaft wont heat up the box like running thru a gearset will I personally think a lot of overdrive transmissions are a breakdown waiting to happen and would rather a higher ratio diff and a direct
drive gearbox but to each his own,

AM

Did you know the Isuzu MSA-D gearbox sold in the NPR's in the US had no over-drive? 5th was straight through.

Originally Posted by debruiser

I understand what you have said. So what you are saying is that if you are towing something heavy, then drive slower? Isnt' that common sense? OR get a stronger gearbox?

You quoted part of my last post when you asked those questions.

I'm unsure if your first question, which I highlighted in red above, is related to my comment on "torque back up" contained in part of the quote from my last post. If so then I am concerned that you may have got a wrong impression.

My intention was to give an example of the engine torque responding to load vs the notion of; the engine produces X amount of torque. It doesn't matter what gear your in it still only produces X torque, ...

For clarification, torque back up is an essential design feature without which it would make many vehicles very hard to drive. For example when more torque is required as the load increases on a hill it makes the increased torque available, i.e. the engine slows and torque increases to cope until, if necessary, the driver can change to a lower gear so the transmission provides the extra torque.

I never meant to imply that the driver lifts their right foot to slow when the load increases. That action would reduce engine torque .........

In my earlier posts, #29 and #33, I was commenting on towing in 5th gear with an LT85 gearbox. I attempted to make the point that the appropriate gear depends on the load on the engine and transmission. And towing or not, the load varies, e.g. up hill, down hill, straight flat open road, tail or head wind, etc., and a rule such as never tow in 5th is not necessary when the load on the engine and transmission is low.

The only thing I want to say about your rhetorical second question is that, common sense is not as common as one may think.

John, I see where your coming from now. So you arn't saying NEVER tow in 5th, which is what I was confused about. I have had many people tell me you should never use overdrive to tow, as a blanket rule uphill, downhill, flat, sideways whatever! So I thought you were 'one of those' but I was wrong. Sorry if I upset you.

Just to restate what you have said so that we're crystal, when you run out of power in 5th change down, rather than labouring in 5th. Really wouldn't that stand true for any gear? ie. if you run out of power in say 3rd, 2nd should be selected.

Originally Posted by steveG

In lower gears you rarely pull maximum engine torque for long as the vehicle will accellerate. You can see this in your EGTs when offroad - they stay pretty low. By comparison you can be near maximum toque for hours on end in a high gear on the highway.

In a 1:1 gear in a normal automotive gearbox there is little radial load on bearings as there is no load transmitted through layshafts etc. By comparison, in an overdrive gear you're definitely able to put full torque through the 5th gear and shafts etc and they often arent built to withstand it.

Thats my take on it anyway

Steve

Originally Posted by Ancient Mariner

Any thing you can do to reduce heat in a transmission has to be a plus and torque being transferred thru a straight shaft wont heat up the box like running thru a gearset will I personally think a lot of overdrive transmissions are a breakdown waiting to happen and would rather a higher ratio diff and a direct
drive gearbox but to each his own,

AM

Two very good posts.

To add my little bit. Overdrive is for conditions when the load is light, when the lower engine rpm can be comfortable for the engine and passengers.

Originally Posted by debruiser

John, I see where your coming from now. So you arn't saying NEVER tow in 5th, which is what I was confused about. I have had many people tell me you should never use overdrive to tow, as a blanket rule uphill, downhill, flat, sideways whatever! So I thought you were 'one of those' but I was wrong. Sorry if I upset you.

Just to restate what you have said so that we're crystal, when you run out of power in 5th change down, rather than labouring in 5th. Really wouldn't that stand true for any gear? ie. if you run out of power in say 3rd, 2nd should be selected.

I've deliberately avoided mentioning power! Power is the time rate of doing work. Work is force (load) x distance. Power is work / time.

With rotary motion:

Work is 2 x pi x torque (force x radius) x number of revolutions - note: 2 x pi x radius is the circumferential distance and multiply by number of revs to get distance
Power is 2 x pi x torque x number of revs per second

Time, therefore power isn't relevant to this discussion.

The so called power band for an engine is the range of engine rpm's between, where maximum torque and maximum power are produced.

Vehicles intended to work for a living (trucks, trains, etc.), as apposed to passenger or recreational vehicles, are designed to run on or close to maximum power, at maximum vehicle speed, all day.

Passenger vehicles are designed with occupant comfort in mind, which is not compatible with running at maximum power. At highway cruising speed when top gear is selected the engine rpm will usually be closer to the low end of the so called power band, particularly those with an overdrive gear.

Typically, gearbox ratios are chosen so the driver can keep the engine in the power band from just after take off to maximum design speed.

First gear needs to provide enough multiplication of engine torque produced just above idle speed, to the wheels so the vehicle can start off from stationary. Most of us will have experienced the need to slip the clutch with a heavy load so that the engine speed can be higher, thus producing more torque than at idle. The so called antistall feature of the modern Defender employs the computer to manage the extra torque when required.

After take off until desired road speed is reached, when engine speed reaches to upper end of the power band you change to the next gear and the engine speed should be near the low end of the power band allowing acceleration to continue.

The converse applies when load increases on a hill. When engine speed is nearing the low end of the power band, shift down to the next gear, ........

If the engine provides more performance than required for normal driving, i.e. producing enough torque for high acceleration, it is likely to be able to accelerate from an engine speed lower than the so called power band.

Getting back to your questions. Yes you should always avoid making the engine labour or lug, in any gear. This is not a good way to treat diesel engines. When they are working hard they need to be able to get rid off a lot of excess heat (heat that wasn't used to produce torque/power) from the area of the combustion chamber. Combustion pressures are very high and the strength of the piston material reduces rapidly at high temperature. A considerable amount of heat is removed from this area in the exhaust gasses, but when labouring the mass flow of air is very low (in relation to higher rpm's), but the combustion heat is very high because the governor is adding the maximum rate of fuel as it thinks the engine speed is too low.

In this situation, selecting a lower gear will simultaneously reduce the load on the engine, so the governor reduces the fuel, thus combustion pressure and heat reduce, and there will be a higher mass flow of air through the engine that removes heat from the combustion area. The engine will love you.

But this thread had quickly became a focus on the strength of the LT85 gearbox and your comment may have in mind something I said earlier (possibly in post #29 about not labouring the engine in 5th.

That comment was specific to the not uncommon failure of the bearing and pinion shaft in the LT85 when fitted behind the Isuzu 4BD1. It will do little harm to the LT85 if it is in 4th (direct drive).

Sorry if I hijacked the thread.

Originally Posted by Dougal

Did you know the Isuzu MSA-D gearbox sold in the NPR's in the US had no over-drive? 5th was straight through.

I was aware of the MSA-D which would probably be ideal ratios for the 3.0-1 diffs but the
$1.04 purchase price of the MXA 5r kinder sealed the deal .Being lazy I am content to use the first 4 gears and 5th if the engine is happy but should work extra well in low range

AM

Originally Posted by Ancient Mariner

I was aware of the MSA-D which would probably be ideal ratios for the 3.0-1 diffs but the
$1.04 purchase price of the MXA 5r kinder sealed the deal .Being lazy I am content to use the first 4 gears and 5th if the engine is happy but should work extra well in low range

AM

I'm sure 5th gear in your LT100 equivalent 5 ton truck gearbox will handle towing around a 2 ton landrover. If not, I'm sure you could sell it and double your money.